1. Field of the Invention
The present invention relates generally to an apparatus for carrying out plasma chemical vapour deposition by which one or more layers of doped or undoped silica are deposited onto the interior of an elongated hollow glass substrate tube, and more specifically, to an apparatus comprising a resonator having a resonant cavity formed essentially cylindrically symmetrically around a cylindrical axis, along which axis the substrate tube is positioned, wherein said resonant cavity is substantially annular in shape with a cylindrical inner wall and a cylindrical outer wall, and wherein said cylindrical inner wall comprises a slit that extends along at least part of a circle around the cylindrical axis into which resonant cavity a microwaveguide extends so that microwaves can exit to the cavity enclosed by the cylindrical inner wall via the aforesaid slit. The resonator surrounds at least part of the substrate tube and can be moved back and forth along the longitudinal axis of the substrate tube. The present invention further relates to a method of manufacturing an optical preform by means of plasma chemical vapour deposition, wherein doped or undoped glass-forming gases are passed through the interior of an elongated glass substrate tube under conditions such that deposition of glass layers takes place.
2. Technical Background
U.S. Pat. No. 4,877,938 relates to a method of manufacturing a preform in which a graphite tube is secured to metal parts of a furnace for absorbing microwave energy leaking from the resonant cavity.
U.S. Pat. No. 4,844,007 relates to a device for providing glass layers on the inside of a substrate tube, wherein the inside of a resonator is provided with a heat-insulating and/or heat reflecting material layer so as to thus effect a more uniform heat distribution along the length of the substrate tube.
US 2007/0 003 197 in the name of the present applicant relates to a method and device for manufacturing optical preforms, in which a protective tube envelopes a substrate tube along substantially the entire length thereof so as to create plasma conditions in the annular space formed between the substrate tube and the protective tube for the purpose of collapsing the substrate tube into a solid rod.
EP 0 554 845 relates to a method and apparatus for outside glass deposition of solid preforms, wherein a protective tube is provided along the entire length of the solid preform, and a furnace surrounds the assembly of protective tube and solid preform, in which furnace a resonator is movable along part of the length of the protective tube.
An apparatus for manufacturing optical fibres is known from U.S. Pat. No. 6,260,510 in the name of the present applicant, which apparatus may be used within that context for manufacturing a preform rod, for example, from which an optical fibre can be drawn. According to a known method of manufacturing such a preform, a straight vitreous substrate tube (comprised of quartz, for example) is deposited with layers of doped silica (for example germanium-doped silica) on the cylindrical interior surface thereof. The term “silica” as used herein is to be regarded to be any substance in the form of SiOx, stoichometric or non-stoichometric, whether or not crystalline or amorphous. This can be achieved by positioning the substrate tube along the cylindrical axis of the resonant cavity and flushing the interior of the tube with a gaseous mixture comprising O2, SiSl4 and GeCl2 (for example).
A localised plasma is concurrently generated within the cavity, causing the reaction of Si, O and Ge so as to effect direct deposition of thus for example Ge-doped SiOx on the interior surface of the substrate tube. Since such deposition only occurs in and in the vicinity of the localised plasma, the resonant cavity (and thus the plasma) must be swept along the cylindrical axis of the tube in order to uniformly deposit the interior surface of the hollow substrate tube along the entire length thereof. When coating is complete, the substrate tube is thermally collapsed into a solid rod having a Ge-doped silica core portion and a surrounding undoped silica cladding portion. If an extremity of the solid rod is heated so that it becomes molten, a thin glass fibre can be drawn from the solid rod, which glass fibre is usually provided with one or more coatings and can subsequently be wound on a reel; the fibre then has a core and a cladding portion corresponding to those of the solid rod.
Since the Ge-doped core has a higher refractive index than the undoped cladding, the fibre can function as a waveguide, for example for use in propagating optical telecommunication signals. It should be noted that the gaseous mixture flushed through the substrate tube may also contain other components; the addition of C2F6, for example, leads to a reduction in the refractive index of the doped silica. It should also be noted that the preform rod may be externally coated with an additional glass layer, for example silica by means of a deposition process, or by placing the preform rod in a jacket tube (comprised of undoped silica) prior to the drawing procedure, so as to thus increase the amount of undoped silica relative to the amount of doped silica in the final fibre.
The use of such an optical fibre for telecommunication purposes requires that the optical fibre be substantially free from defects (for example discrepancies in the percentage of dopants, undesirable cross-sectional ellipticity and the like), because, when considered over a large length of the optical fibre, such defects may cause a significant attenuation of the signal being transported. It is important, therefore, to realize a very uniform and reproducible PCVD process, because the quality of the deposited PCVD layers will eventually determine the quality of the fibres; thus it is desirable that the plasma generated in the resonant cavity be uniform (around the cylindrical axis of the cavity). On the other hand, the costs of the production process of an optical fibre will be significantly reduced if the preform rod can be given a larger diameter, because larger fibre lengths can then be obtained from a single rod. These two objects are difficult to reconcile, however, because an increased diameter of the substrate tube will generally lead to a plasma having a deteriorated rotational symmetry; moreover, such a plasma can only be generated by using a much higher microwave power.
The present inventors have found that particles may enter the resonator, for example small quartz particles that come off the substrate tube when the substrate tube is positioned in the apparatus for carrying out plasma chemical vapour deposition. In addition to that, particles from the insulating material of the furnace, in which furnace the resonator is disposed, may enter the resonator via the two ends thereof. When the aforesaid particles are present inside the resonator, this may lead to a short-circuit, viz. an undesirable discharge, during the creation of plasma conditions, which causes damage to the resonator. In addition, the deposition process must be stopped, which is undesirable.
The present inventors have furthermore found that metallic particles may come off the resonator during the plasma chemical deposition process and subsequently find their way onto the exterior surface of the substrate tube. Such metallic particles are not only harmful to the substrate tube, but also have an adverse effect on the operation of the resonator, as a result of which the plasma chemical vapour deposition process may become unstable. In addition, the presence of small metallic particles in an optical fibre leads to high fibre attenuations at certain wavelengths, which is undesirable.